This invention relates to GaN compound Light Emitting Diodes.
A semiconductor light-emitting diode (LED) comprises: a substrate; a light emitting structure; and a pair of electrodes for powering the diode. The substrate may be opaque or transparent. Light Emitting Diodes which are based on Gallium Nitride compounds generally comprise: a transparent, insulating substrate, e.g. a sapphire substrate. Because of the substantial lattice mismatch between an insulating substrate, e.g., a sapphire substrate, and GaN compounds, it is common practice to provide a thin buffer or nucleation layer on the sapphire which is followed by a layer on which an LED structure is grown. Growth of single crystals on insulating substrates has been studied for over 30 years. Early work included growth of both silicon and III-V compounds on a variety of insulating substrates including sapphire and spinel. In these studies it was determined that use of nucleation or buffer layers reduces the occurrences of imperfections and the tendency towards twinning in the thicker layers grown thereon.
In accordance with one aspect of our present invention, we provide a new and novel structure for overcoming the mismatch of the lattices of a sapphire substrate and the nitride layers that follow. We provide three buffer layers on which we grow a high quality I Gallium Nitride layer as a substrate for growth of the light structure. Our first buffer layer is formed of Indium Gallium Nitride. The addition of Indium to the GaN compound provides a soft material with a superior surface diffusion coefficient. These factors facilitate the formation of high quality materials at the beginning of crystal growth. Since InGaN has a larger lattice constant than the target GaN, our second layer is formed of AlGaN to migrate to the lattice constant of GaN. The final buffer layer is formed of GaN to provide a template for the growth of our high quality I GaN layer which serves as a platform for growth of our light emitting structure 12.